I have a mini-itx and have been playing with it as a device for videos/music. For software I've tried out Freevo and GeexBox, both quite impressive, with some rough edges.
GeexBox is very clever - basically just a frontend for MPlayer, but you get aa full development environment to regenerate your own custom version. But it's limited by its dependence on MPlayer for user interaction. Otherwise good for diskless systems, well supported and easy and quick to set up.
Freevo promises a new version, but its project page is pretty quiet at present. I guess MythTV has supporters, but I never tried it. I prefer Freevo because it uses python, which I'm familiar with.
I'm starting the think the best option for media playback is a modded XBox. 2nd hand units are now very cheap - much cheaper than mini-itx systems. Also quiet with plenty of interfaces, and right form-factor.
I haven't looked at the network media player market for a few months tho'. Surely the media playback equivalent of the Linksys wrt54g can't be far off.